In order for loudspeaking telephone sets to function properly, without the troublesome instability sometimes known as the "Larsen Effect", the loop gain encompassing the acoustical coupling path between receiver and microphone must at all times be less than unity. It is, therefore, necessary that the net gain of the transmit plug receive channels be below a critical value, beyond which the said loop gain would exceed unity and oscillation sets in.
A simple (but crude) solution to such problem is to cut off the transmit channel completely while receiving and vice versa. In U.S. Pat. No. 3,725,585 issued Apr. 3, 1973 to Moniak et al, this is accomplished by comparing the receive and transmit signals and switching the circuit in favour of the channel carrying the larger amplitude.
More sophisticated approaches do not effect complete cut-off of one channel in favour of the other, but utilize analog, active variable gain or loss devices in each of the receive and transmit channels to maintain the net gain below the critical value. The variable gain (or loss) devices are controlled by a signal derived from a comparison between the levels in the transmit and receive channels. Such a system is generally disclosed in commonly assigned U.S. Pat. No. 3,889,059 issued to J. L. E. Thompson et al on June 10, 1975 and titled: "LOUDSPEAKING COMMUNICATIONS TERMINAL APPARATUS AND METHOD OF OPERATION". While this approach eliminates the disadvantages of voice operated switching mentioned supra, it creates yet another problem known in the art as "thump". Thump is the undesirable effect of feedthrough of the control signal into the actual speech path. This effect is difficult to suppress due to the closeness of the lower cut-off frequency (300 Hz) of telephone channels to the frequencies present in the varying control signal. In order to reduce this undesirable interference, balanced circuit configurations at the interface points with the control signal are often necessary. Such circuit configurations invariably require component matching and/or critical adjustments in the circuit and are, therefore, not inexpensive to implement; yet they do not provide complete freedom from the objectionable thump.
In some instances, the control signal may contain frequency components within the voice frequency band, and the problem of thump is aggravated. The present invention, in addition to other advantages, endeavours to mitigate the possibility of injection of the control signal into the speech path by means of a reliable, uncritical method and circuit.